Objective: To define reference parameters for analyzing sagittal balance of the cervical spine in asymptomatic volunteers.
Methods: Prospective study after Bioethics Committee approval. Imaging performed using a low-dose radiographic system (EOS Imaging, Paris, France). The absence of pain was assessed using the Oswestry Questionnaire and VAS. 106 subjects were included of whom 55.66 % were men. The parameters measured were: pelvic incidence, pelvic tilt, sacral slope, thoracic and lumbar curvature, C7 plumb line position and the spino-sacral angle. The C7 slope and new parameters were measured: cranial incidence, defined in relation to the McGregor line and the sella turcica allowing to define cranial slope and tilt, and the spino-cranial angle (SCA).
Results: This study demonstrated a close correlation between the C7 slope and the cranio-cervical system. Economic sagittal balance in the asymptomatic population was defined by a constant SCA angle of 83° ± 9°. To maintain this balance, a spine with a marked C7 slope will present lordosis and vice versa. Cranial incidence is an anatomical parameter characteristic of the cranio-cervical system which makes it possible to analyze the spatial positioning of the head and to predict the desired value of cervical lordosis which is closely correlated to cranial slope.
Conclusion: The C7 slope has a predictive value of the shape of the cervical spine in the sagittal plane. One-third of the asymptomatic population had cervical kyphosis. Our results could be used to study sagittal balance before and after arthrodesis, or cervical prosthesis.